1. Field of the Invention
The present invention relates to a light emitting device having a circuit composed of a thin film transistor (hereinafter, referred to as a TFT), and a method of manufacturing thereof. For example, the invention relates to an electronic appliance mounted with a light emitting display device comprising an organic light-emitting element as a component.
2. Related Art
In recent years, research related to a light emitting device having an EL element as a self-luminous light emitting element has been actively carried out. The light emitting device has also been referred to as an organic EL display or an organic light-emitting diode. Since these light emitting devices have such characteristics as rapid response speed, low voltage, low power consumption driving, which are suitable for displaying moving pictures, they have been attracting much attention for next generation displays including new-generation cellular phones and personal digital assistants (PDAs).
An EL element using a layer including an organic compound as a light emitting layer has a structure in which the layer including an organic compound (hereinafter referred to as an EL layer) is sandwiched between an anode and a cathode. By applying an electric field to the anode and the cathode, electroluminescence is generated in the EL layer. Luminescence obtained from the EL element includes luminescence generated in returning to a ground state from an excited singlet state (fluorescence) and luminescence generated in returning to a ground state from an excited triplet state (phosphorescence).
However, sufficient luminance has not been obtained in the conventional light emitting element having the layer including an organic compound as a light emitting layer.
Light generated in the EL layer is emitted through the anode or the cathode to display images on a display surface. At this moment, the light generated in the EL layer is partially reflected by an interface between different material layers while passing through the various material layers, a substrate, and the like. As a result, there have been problems in which the amount of light transmitting though elements is reduced by several tens percent from initial light emission, and luminance is reduced.
Accordingly, the present inventors proposes an element structure for improving the light-extraction efficiency in patent documents 1 and 2.
Further, since the conventional light emitting element having the layer including an organic compound as a light emitting layer has low light-emitting efficiency, the amount of the current is increased in order to obtain the desired luminance, thereby increasing power consumption. Such high power consumption adversely affects the lifetime of an element. In particular, the half-life in luminance is shortened. Accordingly, there has been a problem in which the stability of the element must be improved.
[Patent document 1] Japanese Patent Laid-Open No. 2002-352950
[Patent document 2] Japanese Patent Laid-Open No. 2002-229482